In optical transmission systems, signals of different wavelengths modulated in each case by the data to be transmitted need to be multiplexed in a single main optical fibre referred to as a connection or transmission fibre, and conversely. Optical systems of this nature are referred to as being unidirectional or monodirectional, if the different wavelengths are propagated in one direction only of the connecting fibre, and as being bidirectional if the connecting fibre transmits different so-called outgoing wavelengths in one direction, and one or more so-called return wavelengths in the other direction. At the present time, use is made of bidirectional optical connection systems in numerous spheres such as for example automatic teletransmission, telephone transmissions or teledistribution networks in which a central station is in communication with peripheral stations formed by subscribers' transceiver apparatus.
Different kinds of optical multiplexer-demultiplexer for wavelength transmission lines or channels on a single bidirectional connection fibre are already known. One of these, intended for a bidirectional connection, which for example has three channels, of which two are outgoing and another for return, is formed by two interferential filters comprising multiple dielectric layers placed in alignment, respectively, with the two extremities of the connecting fibre. Each of the two filters performs a dual function being in the one case, the actual multiplexing of the two outgoing channels by distribution into wavelengths and extraction from the return channel, and in the case of the other filter, the actual demultiplexing of the two outgoing channels by selection following propagation in the fibre and transmission of the return channel into the fibre.
However, a bidirectional optical multiplexing-demultiplexing device of this kind has disadvantages. In particular, the fact that the extraction from the return channel is effected by the same interferential filter for multiplexing of the two outgoing channels, and similarly so far as concerns the transmission of the return channel by the same interferential filter for demultiplexing of the two outgoing channels causes a substantial rejection of the return channel on each of the outgoing channels and consequently increases co-channel interference. As a result, each of the filters performing a dual function cannot perform a correct separation of the return channel from the outgoing channels which is particularly troublesome regarding reproduction of the different data. Furthermore, this device is not produced in a compact form, that is to say with little bulk.
The present invention has as its object to remedy these disadvantages by proposing an optical multiplexer-demultiplexer for a bidirectional connection, which permits a substantial reduction of co-channel interference and is provided in a compact form, that is to say of little bulk.